security assist login is based on a session variable, you will need to know the name of the session being used by security assist, on the login page, double click the authenticate user server behavior, go to the third page and make not of the name of the user ID session variable.
you could add a link on the cart page that will be used to trigger a set session variable to fake login:
<a href="cart.php?guest=1>Shop as guest</a>
on the bindings panel, click the plus button and select URL Variable, set the name to "guest"
the on the cart page, go to the server behaviors panel, click the plus button and select eCart -> General -> Set Session Variable
click the lightning bolt next to trigger and select the guest query string variable.
set the name to be the same session name set by the authenticate user behavior
set the value to 0